This project will investigate how steroid receptors interact with the transcription machinery to activate transcription of specific genes. An 810 amino acid fragment of a recently identified protein called GRIP1 interacts with the hormone binding domain of steroid receptors in an agonist dependent manner. In yeast, GRIP1 serves as a transcriptional coactivator for steroid receptors by interacting with the AF2 transcriptional activation domain of steroid receptors. Characterization of GRIP1 will be continued by first completing the cloning and sequencing of the coding region. The possibility that GRIP1 may interact with other nuclear receptors will be investigated. The functional domains of GRIP1 will be mapped as well as the specific subregions of the steroid receptors that GRIP1 interacts with. Cell and tissue specificity of GRIP1 expression will be determined. The ability of GRIP1 to interact with and serve as a coactivator for steroid receptors in mammalian cells will be examined. Finally, the mechanism of transcriptional coactivation by GRIP1 will be studied by looking for interactions between GRIP1 and known components of the transcription machinery, as well as by initiating studies with an in vitro transcription system.